1. Field of the Invention
This invention relates to systems for measuring liquid velocity and in particular for measuring blood velocity on backscatter Doppler principles, and to detect the presence of bubbles within the blood.
2. Description of the Prior Art
It is known to use ultrasonic devices for measurement of the speed of a liquid flow in general. These devices, most of which use two transducers, or sometimes a single transducer, use the Doppler effect. Devices are also known which apply the same principle to blood flow rate measurement. The known devices are speed meters; see particularly the articles by Franklin and collaborators in "The American Journal of Medical Electronics", 1st term 1966, pages 24-28 and "IRE Transactions of Bio-medical Electronics", January 1962, pages 44-49. As shown in FIG. 1, such ultrasonic devices typically included an ultrasonic transducer illustratively in the form of a crystal 14' that is energized by a generator 12' to emit ultrasonic waves into a conduit 24', whereby it is reflected by the liquid directed therethrough to be sensed by a detector, typically in the form of a crystal 18', the output of which is connected to a receiver 20'. The receiver 20' as will be explained below, detects the received Doppler signal which contains the velocity information. In the particular, illustrative context of this invention, such principles are used to measure the velocity of blood as would be directed through a conduit external of the patient's body. In one illustrative embodiment of this invention, it is contemplated that a heart assist machine would be used to aid the patient's heart and that the blood flow through this machine could be measured in accordance with the teachings of this invention.
The practical application of the Doppler backscatter principle consists of transmitting an ultrasonic beam into the medium whose velocity one wishes to measure and to compare the original frequency with the received shifted frequency. To retrieve the velocity information, a comparison of the scattered signal frequency with the original frequency is made by the receiver 20'. The difference in frequency is related to the flow velocity of the medium. Since the medium is flowing generally in a conduit of known dimension, the velocity information can be translated into total flow rate.
There are two basic aspects to this phenomena; the particle size can be larger than the wavelength of the transmitting ultrasound or it can be smaller, therefore acting as a point scatterer. It is noted that the red cells of blood have a typical diameter of 8 .mu.m thickness of 2 .mu.m, with the wavelength of the 3.13 MHz ultrasonic beam being approximately 480 .mu.m in blood. In the case of red cells, the cell is smaller than the wavelength of the beam and the cell is set into motion and becomes a secondary emitter acting as a point source.
The envelope of the received signal represents the heterodyne coupling of the transmitted carrier with the backscattered signal whose normal frequency has been shifted by the Doppler phenomena. This signal then is amplified, demodulated, audio amplified, processed and displayed as flow rate by the receiver 20', as shown in FIG. 1. The shift in frequency is due to the relative motion of the object with respect to the transmitter and receiver. The frequency shift due to motion of the particle with respect to the transmitter is: ##EQU1## where f.sub.1 = Frequency of the forced particle oscillation V.sub.O = Ultrasound velocity in medium
V = Particle velocity PA1 .theta. = Angle between the ultrasound and the velocity vector PA1 f.sub.c = Ultrasonic carrier frequency PA1 V.sub.O, V, O as indicated above
The frequency shift due to motion of the particle with respect to the receiver is: ##EQU2## where f.sub.1 = frequency of the forced particle oscillation f.sub.2 = New frequency as measured at the receiver
Combining (1) and (2), the total Doppler shift may be exposed as: EQU f = (f.sub.c - f.sub.1) + (f.sub.1 - f.sub.2) = f.sub.c - f.sub.2 ( 3) ##EQU3## The formula can be expanded in a series and only the most important term taken when V.sub.O (1500 m/sec) &gt;&gt; V (&lt;1.5 m/sec at 10 L/minute), to provide the expression: EQU f = 2 Vf.sub.c cos .theta./V.sub.O ( 5)
This is the general formula used in the backscatter Doppler flowmeter design. This signal is difficult to detect since the signal amplitude at this shifted frequency is small and becomes swamped by the direct coupled ultrasonic carrier frequency. Fortunately, the direct radiated ultrasonic wave received is mixed with the backscatter signal in the crystal 18' producing an amplitude modulated signal that retains all the basic information as indicated by formula (5). The receiving crystal 18' converts the ultrasonic energy back into an electrical signal. The amplitude modulated signal, at microvolt levels, is RF amplified, detected, audio amplified, processed and displayed as flow information.
When an ultrasonic device is used to measure blood flow external of the human body and in particular in the situation where such a device is used to measure the velocity of blood flowing to a heart assist device, it is contemplated that a supply of air bubbles may be accidently introduced into a blood flow and subsequently into the patient's body, with the possible result that such bubbles would be introduced via the arterial portion of the patient's cardiovascular system into his brain. Such bubbles tend to block the flow of blood through the smaller arteries, thus leading to possible brain tissue damage and ultimately stroke and death.
Thus, it is a principal object of this invention to provide means for sensing the presence of air or other gaseous bubbles within the blood flow to provide a suitable alarm manifestation indicative thereof, whereby attending personnel may check the heart assist equipment for any malfunction and also take appropriate remedial action with respect to the patient.
The air bubble detection system of this invention differs from that blood flow pressure measurement system as described in U.S. Pat. No. 3,640,271, wherein bubbles of a controlled diameter are introduced into a patient's bloodstream to permit detection by ultrasonic devices at first and second points to provide an indication of the blood velocity, dependent upon the time it took for the bubbles to move away from the first to the second point. At the first point, the bubble is subjected to energy at or near its resonant frequency, and at the second point, the bubbles are detected by subjecting them to ultrasonic waves to excite them to the resonant frequency and detecting at the second point backscattered energy indicative of the passage of the bubbles. The noted U.S. Pat. No. 3,640,271 does not contemplate a first or normal mode of operation, wherein Doppler principles are used for measuring blood flow velocity, and a second mode of operation for detecting the inadvertent injection of bubbles into the bloodstream and providing an alarm indicative thereof. In this regard, the inadvertently introduced bubbles may be of a size detrimental to the patient and the system of this invention operates to distinguish normal and abnormal conditions to provide an alarm indicative thereof.